Do you do many situps, abdominal exercises? I didn't for a while and I found that while riding trials, my belly muscles got really tight at the top and migrated up into/put pressure on my chest area causing all sorts of palpitations, chest pain, feeling of imminent death which triggers anxiety which then makes the tightness even worse!
After doing some plank ab exercises, I regained some control over my belly again and managed to 'relax it out of my chest', this caused some strange little sharp pains at first with the top of my abdomen spazzing out but now, I can breathe better, have more energy and no more messed up heart beats, yay!
This took me ages of going to the doctors for treatments of stomach ulcers and other random things, I was a fat kid and being young and self concious of being fat I remember trying to suck my belly in a lot, which also wouldn't have helped
This guy sums it up for me..